1. Field of the Invention
The present invention relates to a drive device including a reverse preventing mechanism that prevents a roller shaft from being rotated in a reverse direction. The present invention further relates to a sheet conveying device using the drive device, and an image forming apparatus such as a copying machine, a facsimile machine, a laser beam printer, or other similar image forming apparatus, including the sheet conveying device.
2. Discussion of the Background
In an image forming apparatus such as a copying machine, a facsimile machine, a laser beam printer, or other similar image forming apparatus, it is necessary to prevent sheet conveying rollers from being reversed for various reasons. For example, to correct a skew feed of a sheet, a leading edge of the sheet is abut against a nip portion of a pair of registration rollers, and the sheet is conveyed for some distance by sheet conveying rollers disposed upstream of the registration rollers in a sheet conveying direction to form a loop of the sheet. At this time, if a reverse preventing mechanism for preventing the sheet conveying rollers from being reversed is not provided, the sheet conveying rollers are rotated in a reverse direction (i.e., in the direction opposite to the sheet conveying direction) due to a tension of the sheet. As a result, the loop of the sheet disappears, resulting in an inferior sheet skew correction. At worst, the leading edge of the sheet goes back from the nip portion of the registration rollers toward the sheet conveying rollers, so that a mis-feeding of the sheet occurs. Generally, a sheet separation roller is disposed upstream of the sheet conveying rollers in the sheet conveying direction, which rotates in a reverse direction to separate the uppermost sheet from the sheets fed out from a sheet feeding cassette. The above-described inferior sheet skew correction and mis-feeding of sheets are typically caused when a force is exerted in a sheet returning-back direction.
To obviate the above-described problems, a reverse preventing mechanism for preventing sheet conveying rollers from being reversed is provided in an apparatus that performs a sheet conveying operation. FIGS. 1, 2A, and 2B illustrate a background reverse preventing mechanism for sheet conveying rollers used in an image forming apparatus. A roller clutch (one-way clutch) 100 illustrated in FIG. 1 is generally used as a reverse preventing mechanism. In the configuration of the roller clutch 100, each of a plurality of needle pins 120 is disposed in a hole-shaped clearance 140 (FIG. 2A) formed between the circumferential surface of a roller shaft 135 and an outer ring 160 such that the plurality of needle pins 120 contact the circumferential surface of the roller shaft 135. As illustrated in FIG. 2B, the clearance 140 includes a narrow width portion B and a wide width portion C. Regularly, each of the needle pins 120 is biased by a spring 150 toward the narrow width portion B of the clearance 140. The size of the clearance 140 is set such that the needle pin 120 can be prevented from falling through the clearance 140.
In this configuration of the roller clutch 100, when the roller shaft 135 is rotated in a direction indicated by arrow A in FIGS. 2A and 2B (i.e., a counter-clockwise direction), the needle pin 120 moves toward the narrow width portion B of the clearance 140 due to friction between the needle pin 120 and the roller shaft 135. By catching the needle pin 120 in the narrow width portion B of the clearance 140, the roller shaft 135 and the roller clutch 100 are locked as illustrated in FIG. 2B. When the roller shaft 135 is rotated in a reverse direction (i.e., a clockwise direction), the needle pin 120 moves toward the wide width portion C of the clearance 140 by overcoming the biasing force of the spring 150, and thereby the roller shaft 135 rotates freely in the clockwise direction. The outer ring 160 is disposed between the roller clutch 100 and a holder 130, and has a function of positioning the holder 130 relative to the roller clutch 100.
FIG. 3 is a schematic view of a sheet conveying roller mechanism provided with the background reverse preventing mechanism for use in an image forming apparatus. As illustrated in FIG. 3, a plurality of sheet conveying rollers 103 are attached on a roller shaft 135 rotatably disposed between side plates 131 and 132 via bearings 133 and 134, respectively. A plurality of driven rollers 137 attached on a driven shaft 136 contact the sheet conveying rollers 103 with a predetermined pressure, respectively, to convey a sheet without fail by applying a frictional force to the sheet at the nip portions between the sheet conveying rollers 103 and the driven rollers 137. As illustrated in FIG. 3, the roller clutch 100 is often used as a bearing by pressing the roller clutch 100 into the bearing 134 that supports one side of the roller shaft 135.
In the above-described sheet conveying roller mechanism provided with the roller clutch 100, the following problems typically arise. Generally, to convey sheets, the sheet conveying rollers 103 convey sheets by use of a frictional force produced by contacting the driven rollers 137 with the sheet conveying rollers 103 with a predetermined pressure as described above. Accordingly, a load is generated in the roller shaft 135 in its radial direction. Thereby, the roller shaft 135 is pressed against the needle pins 120, and the needle pins 120 are pressed against the outer ring 160 facing the clearance 140. The rotation and halt of the roller shaft 135 are repeated in this condition.
Although a rust preventing oil is applied to the needle pins 120, it is difficult to use a lubricating oil because a slip occurs at the time of halt (locking) of the roller clutch 100. The needle pins 120 are generally formed from a hard material of iron and steel and are subjected to quench hardening in view of the needle pins 120 abrading over time. In the above-described use conditions of the roller clutch 100, the roller shaft 135 wears over time, and abrasion powders accumulate in the roller clutch 100. The diameter of the roller shaft 135 decreases, and/or abrasion powders enter portions between the roller shaft 135 and the needle pins 120. As a result, the roller shaft 135 becomes unable to be locked, an abnormal noise is produced, and a useful life of the roller shaft 135 decreases. The roller shaft 135 may be subjected to quench hardening to extend its useful lifetime. However, because the roller shaft 135 needs to be formed from a special stainless material free from rusting, subjecting the roller shaft 135 to quench hardening increases the cost of the roller shaft 135. Further, as the length of the roller shaft 135 is generally greater than a width of a sheet, the cost of the roller shaft 135 increases that much more.
In addition to the above-described problems, in a high-speed apparatus in which a rotational speed of the roller shaft 135 is high, heat produced by friction between the needle pins 120 and the roller shaft 135 and heat produced by friction between the needle pins 120 and the outer ring 160 facing the clearances 140 increase. As a result, seizing up of metallic members typically occurs, and an abnormal noise tends to be produced.
In an image forming apparatus, the sheet conveying rollers 103 and an image forming device are generally disposed between the pair of side plates 131 and 132. A drive system such as gears, motors, and clutches is disposed at the outside of one of the side plates 131 and 132. Such a drive system may be disposed on each side of the side plates 131 and 132 instead of one side thereof. However, because wires and gear trains used for a motor and reduction gears need a predetermined space for their layouts, if such a drive system is disposed on each side of the side plates 131 and 132, the width of the apparatus increases. Therefore, the layout of a drive system concentrates on one side of the apparatus.
In a recent space-saving printer or multi-function apparatus, the space for the layout of units is restricted, and therefore some units need to be disposed on a side opposite from a drive source relative to a sheet conveying area. For example, a waste toner tank may be applied to this case. The waste toner tank is preferably disposed at the outside of one of the side plates 131 and 132 for easy maintenance. On the side where a drive system is provided, a motor, a structure for supporting the motor, and shafts for transmitting a drive force of the motor to each unit are disposed. Further, a waste toner conveying path for conveying waste toner from an image forming device, which is disposed between the side plates 131 and 132, to the waste toner tank needs to protrude through one of the side plates 131 and 132. Therefore, it is difficult to dispose the drive system and the waste toner tank on the same side of one of the side plates 131 and 132.
To drive a unit such as a waste toner tank disposed on a side opposite from a first drive source such as a motor, a second drive source needs to be provided on a side opposite from the first drive source. In this case, it costs extra money for a motor, a driver, attaching parts, and electric wires. Further, measures against waves and noises produced from electric wires and motors need to be taken on the both sides of the side plates 131 and 132.
Moreover, to enhance maintenance of a waste toner tank in an image forming apparatus, it is desirable that the waste toner tank is disposed at the outside of a side plate located on a side opposite from a drive system. This location of the waste toner tank is convenient for a user who replaces the full waste toner tank. Generally, the maintenance of a drive system is conduced by a customer engineer for safety. Further, a user replaces a photoreceptor/cleaning unit (PCU) and an intermediate transfer belt unit, for example, by opening a cover disposed on a side opposite from the drive system. It is preferable that the number of covers to be opened for maintenance is reduced to a minimum. Therefore, the same cover to be opened for maintenance is preferably shared among the waste toner tank, the PCU, and the intermediate transfer belt unit.